What is DNS and Web Addresses.?
An Internet or Web address is used to view a web page. When you are viewing a Web page, the web address of the page appears in the Address bar in the browser. In the previous section, you have gone through the concept of IP addresses. What do you think about the IP address? Are they not very cumbersome to remember? For example, to visit IGNOU website the address www.ignou.ac.in is far simpler than that of an IP address like 190.10.10.247. Obviously, what we want to use are simpler textual domain addresses instead of complex IP addresses. However, to enable the use of simple textual address, you will require a service that will map these text based names to respective IP addresses automatically. Such a service was designed in 1983 by the University of Wisconsin with the name Domain Name System (DNS).
In the present day, Internet, Domain Name System (DNS) should keep track of address of each computer or any other internet device and email addresses. The name servers translates the web address or email address to respective IP address. For example, the name server translates address like www.ignou.ac.in into a computer understandable IP address. It sounds simple, but remembers on Internet you are dealing with million of addresses and every day this list is increasing. All these computers have a unique address. Therefore, DNS follows a hierarchical naming scheme that is supported by distributed database system to ensure no duplicate names are issued at all. Figure shows the hierarchical structure of domains names on Internet. For example, traversing the hierarchy from the top you can track down ignou.ac.in as: First you can find the in (India) in the top level country domains. Within this domain find the ac (Academic) sub domain. Please note most of the Indian Universities will be in this sub-domain. Finally, in the ac you can find the entry for ignou. This entry should point to the IP address for the ignou.ac.in for the WWW as well as for the mail server. This is how the DNS finds the addresses, thus, is a very efficient system.
Thus, using the DNS you will be able to relate a given textual address to IP address. For converting domain name into IP address, it first accepts request from programs and other servers. After accepting the request, the name server can do the following:
If it knows the IP address of requested domain, it will answer the request with an IP address of the requested domain.
If it does not know the requested domain name, it will contact another name server and try to find the IP address.
If the requested domain name is invalid or domain does not exist, it will return an error message.
But how can you name a web page on the Internet? To answer this question you may first identify that a web page actually is part of a website that may reside on a web server having a unique IP address. Thus, to identify a web page you need to identify –
The protocol used to access that page.
The server on which the website is located.
The name of the page within that web site. Please note that simple web pages are stored as files.
Please note that a web page may be stored as a single or multiple files. Thus, to identify a web page you will have an address like:
the address as above recognises the protocol http (Hyper Text Transfer Protocol) to access the page,
the name of the page accessed by you is result.html which resides in the students folder within the website.
This address is called the URL. URL stands for Uniform Resource Locater. You can now clearly see that a URL consists of three parts – the first part is used to tell the browser what kind of server it will connect to. In the example above, the browser will connect to a web server using Hypertext Transfer Protocol (HTTP). Other protocols that we can use in this field of an URL are FTP, smtp etc. the protocol is always followed by ―://‖.
The second part of an URL is a fully Qualified Domain Name (www.ignou.ac.in). In an URL, the fully qualified domain name identifies the site running the server. Web servers use port 80 by default, but some servers has been set up to use other ports. For this, a URL can contain a port number following the domain name and separated from it by a colon (www.ignou.ac.in:80), it is optional to write a port number with domain name. If the URL contains no port number, the default port is used. The first two parts of an URL are used to identify the web server of the website. Each web server has a home page and a directory to store the entire document related to the web page like images, audio, video files.
The third component of URL is an optional pathname for a particular document itself. For example, the address http://www.ignou.ac.in/students/result.html specifies the file result.html i.e., in the directory students (/students/result.html) in the specified web server.
But how does this information exchange between the web client and web server is achieved? This whole communication is managed by a protocol called the Hyper Text Transfer Protocol (HTTP). However, the only protocol that works on Internet as told to you in the previous section was TCP/IP. So what is this HTTP? Please note HTTP can work only over a connection that is managed by TCP. Thus, it is a higher level protocol that uses the services of TCP.
HTTP specifies the list of actions that lead to transfer of a requested information exchange between a web client and web server. Whenever you wish to visit a web page on the internet, you request that page from a web server. When you type a URL into your browser (for example, "http://www.abc.com/"), your web browser requests the page (or file) named index.html from the web server and the web server sends the page back to the web browser. Let us identify these steps in more details:
1) As a first step you may put a URL like http://www.abc.com/index.html or equivalent Domain name www.abc.com as the address of the website that you want to access through your web browser.
2) The Web browser tries to resolve the IP address of the website www.abc.com by the information available in its own cache memory. If web server does not have the information about IP address stored in its cache, it requests the IP address from Domain Name System (DNS) servers. The DNS server tells the browser about the IP address of the website.
3) Once the web browser knows the IP address of the website, it then requests the web page (index.html page which is the home page in the present example) from the web server.
4) The web server responds by sending back the requested web page. If the requested page does not exist then it will send back the appropriate error message.
5) Your web browser receives the page from the web server and displays it as per the display requirements of the web page.
What is E-mail Addresses.?
As you have studied earlier that e-mail is one of the popular services increasingly being used by people in their daily life. The following can be a typical email address format on Internet for any e-mail service provider like, Gmail, Rediff, Yahoo, MSN, or any network (domain) name etc.
The username in general is the name assigned or chosen during creation of an email account. Sub-domain are domain we have already discussed in above section, in case of private service provider it is generally its own name like abc@yahoo.com, abc@yahoo.co.in, abc@gmail.com, etc. On the Internet you can see both kind of domains non-Geographic and geographic domains. Lets take an example to better understand an e-mail address: In an e-mail address "naveen@ignou.ac.in", naveen indicates the username, the sub-domain named IGNOU (Indira Gandhi National Open University) which is an academic organisation (.ac) and is situated in country India (.in).
Check Your Progress 3
1. What are the services on Internet?
Today you can avail the facilities of e-mail; messenger services, Chatting etc., to share your ideas, knowledge, and feeling. You can join different groups, discussion forums or create your own blogs. You can use websites to broadcast huge amount of information on Internet.
2. What is firewall? Where can it be used?
Firewall is software that works on some set of rules and instructions given by you. A firewall helps to keep your computer more secure and protect from many security problems like; hacking, Trojan Horse, Virus, etc. It restricts information that comes to your computer from other computers, giving you more control over the data on your computer and providing a line of defense against people or programs (including viruses and worms) that try to connect to your computer without invitation.
3. What it TCP/IP? Why is it used?
A stack of protocols called TCP/IP (Transmission Control Protocol/Internet Protocol) implements different rules to handle the data communication from source machine to destination machine. For sending a message from source machine to destination machine, TCP divides the message data into little data packets. It also adds special information e.g., the packet position, error correction code etc., to make sure that packets at the destination can be reassembled correctly and without any damage to data. The role of IP here is to put destination-addressing information on such packets. On Internet it is not necessary that all the packets will follow the same path from source to destination A special machine called routers tries to, load balance various paths that exist on networks. Other special machine called gateways allows different electronic networks to talk to Internet that uses TCP/IP.
4. What is a URL?
A URL is a unique identifier for a resource on Internet.
5. Define the terms DSN, IPv4 address, Subnet mask.
DNS is responsible for Web addresses to IP address; IPv4 address is a 32 bit address of a host on Internet. Subnet mast separates Network ID and Machine ID.
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